Next week I'll go to California, and I need to connect, with my phone, to the web without limits. I have a quadband phone with Gprs and Umts.
I see the vary prepaid plans of At&T and T-Mobile.

T-Mobile offer this:

"Sidekick® rates

Get unlimited e-mail, Web browsing, domestic instant messaging, and domestic text messaging for $1 per day, and spend only $0.15 per minute for nationwide calls."

So the question: is this the best plan there are in usa to connect without limits? Is possible to buy only sim? How to activate the plane? Hot to config the phone? On nokia site the us configurations are not ok...
And finally others plane and what are their limits?

The prepaid Sidekick plan is pretty much the only prepaid I know of with AYCE (unlimited) data. You might want to check HowardForums.com and search for prepaid and Sidekick to learn some of the trick to getting this plan set up. I'm not really familiar with it.

T-Mobile ToGo prepaid in general used to provide essentially unlimited tetherable GPRS access, but it was an error and not intended. The system has since been fixed to only allow basic WAP access and only to certain sites. The Sidekick can access any WAP site.